Mass production of multi-component plastic items, such as, e.g., generally tubular toothbrush handles and other similar multi-component plastic elements, are typically made by a multi-step injection-molding process, wherein multiple molding steps are performed at multiple injection-molding stations. In the context of mass production of identical articles, those multi-component plastic elements, which will later become part or parts of the finished articles, are required to have a certain size and shape uniformity. This uniformity can be defined by the extent to which minute variations in corresponding shapes and sizes among the identical parts being successively injection-molded can be tolerated. The concern for uniformity is particularly important when the manufacturing process requires various molds to be involved—and becomes even more pronounced when multi-component parts that are required to be virtually identical are manufactured at multiple locations, which may have somewhat different manufacturing conditions as well as equipment and suppliers of the plastic material.
Virtually all plastic materials, after having being heated to be liquefied—and then cooled and solidified, typically shrink, thereby reducing their physical dimensions. This phenomenon is commonly referred to as “mold shrinkage.” Since identical or similar plastic materials are expected to shrink proportionally to the same or similar degree, plastic parts having relatively greater dimensions shrink, in absolute numbers, to a greater extent relative to parts having relatively smaller dimensions. At the same time, while a shrinkage rate or percentage of shrinking for a certain material, such as, e.g., polypropylene (PP) or polypropylene (PE), can generally be known, it may be difficult to accurately predict the exact mold shrinkage beyond a “typical” shrinkage rate known for these materials. And the greater the dimension of the plastic material subjected of shrinkage, the more difficult it is to precisely predict the exact amount of shrinkage. This difficulty can be attributed to the following factors.
Shrinkage of a plastic part made by molding is believed to be likened to linear thermal contraction or expansion. When a mass of molten polymer is subjected to cooling, it contracts as the temperature drops. Holding pressure may be used to minimize shrinkage—but this can be effective only as long as the gate(s) remains open. If the polymer is homogeneous, all parts are expected to shrink proportionally even after the pressure is removed or the gates freeze off. This is what generally occurs with amorphous polymers, such as, e.g., polystyrene, polycarbonate, ABS, et cetera.
But PP and PE typically behave differently. Unlike amorphous polymers, PP and PE are not homogeneous materials—but are, instead, semi-crystalline materials, having a structure containing both amorphous components and crystalline components. Crystals normally shrink at rates higher than the rates at which the amorphous components shrink. Therefore, as these semi-crystalline materials, containing both amorphous and crystalline components, cool and solidify, they shrink at different rates. This imbalance typically results in a net increase in shrinkage and introduces sensitivity to molding parameters that may have additional effects on the shrinkage.
Another factor influencing shrinkage is believed to be linked to the viscoelastic characteristics of high-molecular-weight polymer melts in a mold. Long molecular-weight chains are stretched in the mold—and thus experience stress therein. During subsequent cooling, this stress is relieved, and the chains tend to relax. This relaxation influences the shrinkage, especially in differential flow directions. Both the average molecular weight and the molecular-weight distribution impact this aspect of mold shrinkage. Other variable factors that may influence shrinkage include thermal history of the molding, e.g., the melt temperature and cooling rate, as well as a thickness of the part being molded, gate dimensions, and other relevant factors.
In addition, plastic parts having a complex geometry, and especially those parts that comprise multiple layers of different plastic materials, tend to have differential shrinkage rates in different sections of the part. Although this phenomenon is largely pronounced during molding of parts having differential wall thickness, it may occur even in parts having relatively uniform wall thickness. The latter can be attributed, among other things, to non-uniform cooling and/or non-uniform filling patterns.
During a molding process of a multi-component part, such, e.g., as a toothbrush-handle housing or a power-tool housing, involving different molds, it may be necessary to place the part being made at different fixation geometries, i.e., mold cavities and/or mold cores. Molds having cores are naturally required for producing molding parts having a generally tubular geometry. A change of mold cavities is typically required to form a tubular part having multiple layers or components made of multiple plastic materials. A change of cores can also be required if one wishes to add one or more characteristics, geometries, or components to the inner surface of the tubular part being made, i.e., an area that is in contact with the core. Another reason for the change of the core in some instances may be dictated by a requirement that a following molding step is to be conducted on additional molding equipment incorporating another core.
If the part molded on a first molding tool needs to be transferred to a second molding tool for further molding/overmolding, this part's positioning in the second molding tool needs to be exact, allowing for very small tolerances. As used herein, the term “tolerance” refers to an allowable amount of variation of a specified measurable dimension, particularly a length dimension of a multi-component housing or any part thereof. As no item or any of its parts can be produced having dimensions precisely to the exact nominal value, tolerances are typically assigned to parts for manufacturing purposes, as boundaries for acceptable build.
Hence, there are acceptable degrees of deviation from the exact nominal value, suitable for a particular machine, process, or part. A manufactured part having dimensions that are out of tolerance will be unlikely a usable part for the intended purpose. Tolerances can be applied to any dimension. In the present context, lengthwise tolerances of a plastic part of parts, made by an injection-molding process, are of a particular interest. The exact positioning of the plastic part inside a mold is required to enable a reliably stable process and accurate touch-up lines between various molded components. The latter may greatly influence functional and aesthetic aspects of the finished product.
When the plastic part molded in one mold cavity is transferred to another mold cavity for further overmolding by another plastic material, one needs to take into account the fact that the length of the molten plastic part will likely change after it is cooled and solidified—as a result of the plastic material's shrinkage caused by cooling. If the shrinkage is significant, resulting in the solidified part being too short for a particular mold cavity, the mold parts intended to contact the part's surfaces may not reach those surfaces to provide a secure contact therebetween. The resulting undesirable empty spaces between the mold parts and the part's surfaces will likely cause “flashes” of the plastic material that is subsequently molded over the part that is too short.
Reversely, if the part being molded is too long for a subsequent mold cavity, the compression caused by the mold cavity's surfaces in contact with the part may crush the edges of the part being made. In addition, the mold may not be able to close completely and securely if the part disposed therein is too long for this mold. The latter may also lead to problems with the mold tool itself, including its premature ware and damage.
The mass production of the increasingly complex molded parts, such as, e.g., a multi-component handle part for a power toothbrush, requires successive changes of molds and/or mold cores. Such changes are required, e.g., when a previously molded part comprising a first plastic material needs to be at least partially overmolded with at least a second plastic material; and then the composite part, comprising the first and second plastic materials, needs to be further at least partially overmolded with at least a third plastic material, and possibly fourth plastic material, and so on.
These multiple successive molding steps require an exact positioning of the part being manufactured (i.e., molded/overmolded) step-by-step in every mold cavity and/or mold core that needs to be used in the process. To accomplish such an exact positioning, the manufacturer needs to make sure that the size and geometry of all elements, including the part being manufactured and the mold components used, match one another with a high-level precision, allowing for very small tolerance. These tolerances are often hard to achieve, particularly with respect to plastic parts affected by shrinkage, as is explained herein.
The exact positioning of the part being manufactured is particularly important in the context of a mass production that may take place at different locations. Such mass production requires multiple identical mold tools that are typically installed on different molding machines—all of which are intended for making identical parts. For example, for the injection molding of a power brush's handle, which is designed to house a motor, a battery, and electronics, as well as to have other functional attributes, the reliable uniformity and precision among the different molds, as well as the handle parts being made on those molds, are of high importance.
Therefore, current molding processes, which require a high degree of precision with respect to dimensions of a part or parts being made, tolerate only limited size and shape variations among the parts being manufactured. For example, a typical current injection-molding process used in the production of power-toothbrush handles is particularly sensitive to length variations of the plastic components. During cooling these components shrink, in absolute numbers, to a much greater extent in their lengthwise dimensions than they do in their dimensions extending perpendicularly to the lengthwise dimensions—due to the fact that their lengthwise dimensions are several times greater than their largest dimension perpendicular to their lengthwise dimensions.
For example, for some typical embodiments of the toothbrush handles, e.g., those having an overall length in the range of about 120-200 mm, and more specifically in the range of about 140-180 mm, the current molding process allows for lengthwise tolerance of not greater than ±0.2 mm in most of the successive molding operations. This may be difficult to maintain uniformly, particularly given the combination of all the factors and concerns described herein above.